Chapter 1: Introduction to PLCs Computer Aided Manufacturing TECH 4/53350 1
Intro to PLC Learning objectives Two ways to categorize a control system Overview of PLCs Differences between PLC, relay and PC-based control Basic PLC architecture Computer Aided Manufacturing TECH 4/53350 2
Automatic Control in Manufacturing Why Automatic Control is necessary To improve the quality and lower the cost of production. To increase the production rate. To attain optimal performance. To relieve the drudgery of many routine, repetitive manual operations: Metal matching sequences Product assembly lines Batch chemical processes Profitability usually depends on productivity and automation is a means towards greater productivity Computer Aided Manufacturing TECH 4/53350 3
Automatic Control System Basic Elements Input Sensors Measurements (Electrical signals) Programmable Output Logic Controller Actuators Resultant Actions Input Sensors: Convert physical phenomena (for example, position) to electrical signal. Output Actuators: Convert electrical signal to a physical action, for example, air valve or motor. Programmable Logic Controller: Using the measurements, calculates control actions. Computer Aided Manufacturing TECH 4/53350 4
Programmable Logic Controllers Mj Major Types Programmable Logic Control (PLC) Developed initially out of Automotive Industry Distributed Control System (DCS) Employed in the Chemical Industry DCS and PLC systems are merging into one device: Programmable Electronic System (PES) Our focus is on PLCs. We will talk more about this shortly Computer Aided Manufacturing TECH 4/53350 5
Automatic Control System Major Types of Sensors/Actuators Discrete Control Systems: Dominated by Discrete Sensors and Actuators: Take one of two values: On/off, Open/Closed, running/stopped, extend/retract Analog Control System: Dominated by Analog Sensors and Actuators Take an infinite number of values (continuous stream) Position, acceleration, temperature, flow Computer Aided Manufacturing TECH 4/53350 6
Automatic Control System Continuous Vs. Discrete Control Most real programmable logic control systems are a combination of continuous and discrete sensors and actuators. Discrete: Analog: Discrete elements: proximity it switches (sensors) control valve for air solenoid (actuators) Continuous (Analog) elements: air flow sensor command speed to fan motor controller Computer Aided Manufacturing TECH 4/53350 7
Automatic Control System Type of Process Industrial Manufacturing processes are classified based on the behavior of the actuator s output: Continuous Process Material passes in a continuous stream through the processing equipment: Steel Rolling Mill Computer Aided Manufacturing TECH 4/53350 8
Automatic Control System Type of Process (Batch) Batch Process Actuators produce Finite quantities (batch) of materials. The input materials typically assume a defined order of processing actions Computer Aided Manufacturing TECH 4/53350 9
Automatic Control System Type of Process (Discrete) Discrete-Parts Manufacturing A specified quantity of materials moves as a unit (group of parts) between workstations, on an assembly line/conveyor belt. Each unit has a unique id. A unit may be modified (drilled, painted etc) at a workstation. What is the output of the actuators? Computer Aided Manufacturing TECH 4/53350 10
Programmable Logic Control PLC PLC is an Industrial Computer Hardware & Software adapted to Industrial Environment and Electrical Technician Why Programmable Logic Control? PLC is the work horse of industrial automation Production processes go through a fixed repetitive sequence of operation with logical steps Electronic Relays were used prior to PLCs: Control systems were hard-wired using relays, timers and logical units Control system had to be re-wired for new applications Inflexible and time consuming Resulted in product delays, high production costs Computer Aided Manufacturing TECH 4/53350 11
PLC Overview Data Memory Output Device Di Driver Process actuators Control Unit Program Memory Input Device Driver Process sensors Programmable Module (PC) Computer Aided Manufacturing TECH 4/53350 12
PLC Overview A PLC consists of 4 main parts: Program Memory: Stores instructions for logical control sequence Data Memory: Stores status of switches, interlocks, past and current values of data items Output t Devices: Hardware/software drivers for industrial i process actuators Solenoid switches, motors, valves Input Devices: Hardware/software drivers for industrial process sensors Switch status sensors, proximity detectors, interlock settings Central Processing Unit: Brain of the PLC Computer Aided Manufacturing TECH 4/53350 13
PLC Components Power Supply Switches Sensors Smart Devices Input Section Central Processing Unit (CPU) Output Section Motor Starters Lights, Valves Smart Devices Memory Programming Unit Protects CPU from Real-world input
PLC Components Central Processing Unit (CPU) CPU Contains one or more processors to control the PLC Handles communication with other components Handles computations: executes OS, manages memory, monitors inputs, evaluates the user logic ( ladder logic ) Programming language: ladder logic Lab PLC CPU (Rockwell) Keyswitch is used to switch between different CPU modes Program Mode Run Mode Remote Mode
Lab PLC CPU Modes Program Mode: All outputs are forced to off condition regardless of their state in logic: Develop and download your program in memory Run Mode: PLC continuously scans and executes your ladder logic Remote Mode: Allows you to remotely control the CPU mode from your computer: Switch between Program and Run modes from your computer Computer Aided Manufacturing TECH 4/53350 16
PLC Components Memory PLC Memory stores OS memory and application memory OS Memory: The OS is burned into ROM by manufacturer and controls system software used to program PLC ROM is non-volatile Application Memory: Stores status of inputs and outputs, i.e. I/O Image tables as patterns of 0 and 1 (binary digits) Stores contents of variables in user programs: timers, counters Random Access Memory: Your programs run/execute in the RAM. RAM is volatile All data items in the RAM are lost if power is turned off Mode
PLC Input/Output Modules Input Module: Takes inputs from the outside world from any device (protects the CPU) Converts real-world logic to CPU logic 250 VAC Device (250 VAC Input Module) low-level level DC Signal Identify the input module in your lab PLC; How may inputs does it take? Output Module: Provides connection to the real-world output devices Output t modules can handle DC or AC voltages to output t digital it or analog signals Identify the output module in your lab PLC; How may output devices does it take? Computer Aided Manufacturing TECH 4/53350 18
Basic PLC Architecture Computer Aided Manufacturing TECH 4/53350 19
Control System Hardware A Comparison Characteristic Relay Systems Computers PLC Systems Price per func. Physical size Oper. Speed Industrial environment Design Complicated operations Installation Easy to change function Ease of maintenance Recovery from power failure Fairly low Low Low Bulky Fairly compact Compact Slow Fairly fast Fast Excellent Fair to good Good Time-consuming Usually simple Simple No Yes Yes Time-consuming Simple to complex Simple Very difficult Usually simple Very simple Poor - many contacts Fair - several custom boards Good - few std. cards 0 sec. 1-100 sec. 1-3 sec. Computer Aided Manufacturing TECH 4/53350 20
PLCs Vs. Computers The architecture of the PLC is basically the same as of a general-purpose computer. Industrial Environment - Can be placed in areas with substantial amount of: Electrical noise Electromagnetic interference Mechanical vibration Extreme temperatures (140 F) Non-condensing humidity (95%) Reliability Built to run continuously for years. (Compare with Windows OS) Easily maintained by plant technicians - Hardware interfaces easily connected. Modular and self-diagnosing interface circuits pinpoint malfunctions and are easily replaced. Programmed using ladder logic, easy to learn. Executes a single program in orderly and sequential fashion - Most medium to large PLCs have instructions i that allow subroutine calling, interrupt routines, and bypass of certain instructions. Also, many PLCs can have modules that implement higher-level languages, e.g., BASIC. Recovers quickly from power failure No boot-up procedure. Diagnostic self- tests and resumes running. Computer Aided Manufacturing TECH 4/53350 21